Deafness | Hearing loss | classification and causes
Deafness is defined as a degree of impairment such that a person is unable to understand speech even in the presence of amplification. In profound deafness, even the loudest sounds produced by an audiometer (an instrument used to measure hearing) may not be detected. In total deafness, no sounds at all, regardless of amplification or method of production, are heard.
Hearing loss exists when there is diminished sensitivity to the sounds normally heard. The term hearing impairment is usually reserved for people who have relative insensitivity to sound in the speech frequencies. The severity of a hearing loss is categorised according to the increase in volume above the usual level necessary before the listener can detect it.
Another aspect of hearing involves the perceived clarity of a sound rather than its amplitude. In humans, that aspect is usually measured by tests of speech perception. These tests measure one’s ability to understand speech, not to merely detect sound. There are very rare types of hearing impairments which affect speech understanding alone.
Types of Hearing Loss
Conductive hearing loss
A conductive hearing loss is caused by any condition or disease that impedes the conveyance of sound in its mechanical form through the middle ear cavity to the inner ear. A conductive hearing loss can be the result of a blockage in the external ear canal or can be caused by any disorder that unfavorably effects the middle ear’s ability to transmit the mechanical energy to the stapes footplate.
Sensorineural Hearing Loss
Sensorineural hearing loss results from inner ear or auditory nerve dysfunction. The sensory component may be from damage to the organ of Corti, an inability of the hair cells to stimulate the nerves of hearing or a metabolic problem in the fluids of the inner ear. The neural or retrocochlear component can be the result of severe damage to the organ of Corti that causes the nerves of hearing to degenerate or it can be an inability of the hearing nerves themselves to convey neuro chemical information through the central auditory pathways.
Mixed Hearing Loss
A mixed hearing loss can be thought of as a sensorineural hearing loss with a conductive component overlaying all or part of the audiometric range tested. So, in addition to some irreversible hearing loss caused by an inner ear or auditory nerve disorder, there is also a dysfunction of the middle ear mechanism that makes the hearing worse than the sensorineural loss alone.
Central Hearing Loss
Central hearing loss is caused by a problem with the auditory nerve or sound centres. Sounds waves may travel through the ear but this nerve pathway is unable to send electrical impulses to the brain. As a result the hearing centres do not receive the signals correctly. Central hearing loss can be a result of a head injury or disease. A common symptom is the ability to detect sound but not being able to understand it.
What is Tinnitus?
Tinnitus is described as any sensation of sound in the absence of any external stimulation. It is often described as a ringing, buzzing or pulsing sensation in the ears. Tinnitus is thought to be caused by damage to the outer, middle or inner ear or the hearing nerve in the brain.
The damage to the hearing system can result from a number of different sources, such as; noise exposure, age-related changes in the inner ear, certain medications, head injuries, wax, etc. If you experience ongoing tinnitus contact our clinic to schedule an appointment for a complete audiological assessment. Be sure to indicate that you are booking the test because of your tinnitus.
Dizziness is a common complaint from many clients, especially those over the age of 70. Benign paroxysmal positional vertigo (BPPV) is the most common cause of positional vertigo. Vertigo refers to the perception of movement (i.e. spinning or turning).
The words: benign, paroxysmal and positional all describe the type of vertigo. ‘Benign’ refers to the idea that it is not life-threatening and there is no known cause; ‘paroxysmal’ suggests that there is a common cycle to the response and that it goes away and ‘positional’ refers to the fact that the vertigo is a result of a particular head or body movement.
Symptoms of BPPV include dizziness when lying down, rolling over or changing head positions. The perception of movement usually lasts less than a minute and is sometimes accompanied by a longer lasting feeling of nausea.
There is a progressive loss of ability to hear high frequencies with increasing age known as presbycusias. This begins in early adulthood, but does not usually interfere with ability to understand conversation until much later. Although genetically variable it is a normal concomitant of aging and is distinct from hearing losses caused by noise exposure, toxins or disease agents.
Noise-induced hearing loss. Noise is the cause of half of all cases of hearing loss, causing some degree of problems in 5% of the population globally. Populations living near airports or freeways are exposed to levels of noise typically in the 65 to 75 dB (A) range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing.
The government departments and some policymakers have set noise standards to protect people from these adverse health risks. The NEPA has identified the level of 70 dB(A) for 24 hour exposure as the level necessary to protect the public from hearing loss and other disruptive effects from noise, such as sleep disturbance, stress-related problems, learning detriment, etc.
Hearing loss can be inherited. Both dominant and recessive genes exist which can cause mild to profound impairment. If a family has a dominant gene for deafness it will persist across generations because it will manifest itself in the offspring even if it is inherited from only one parent.
If a family had genetic hearing impairment caused by a recessive gene it will not always be apparent as it will have to be passed onto offspring from both parents. Dominant and recessive hearing impairment can be syndromic or nonsyndromic. Recent gene mapping has identified dozens of nonsyndromic dominant (DFNA#) and recessive (DFNB#) forms of deafness.
Measles, Meningitis, Autoimmune disease, mumps, Presbycusias, Chlamydia, syphilis, premature birth, Fetal alcohol syndrome, etc.
Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the amino glycosides (main member gentamicin) and platinum based chemotherapeutics such as cisplatin. Some medications may reversibly affect hearing. This includes some diuretics, aspirin and NSAIDs, and macrolide antibiotics. Others may cause permanent hearing loss. Extremely heavy hydrocodone use is known to cause hearing impairment.
It is estimated that half of cases of hearing impairment and deafness are preventable. A number of preventative strategies are effective including: immunization against rubella to reduce congenital infections, immunization against H. influenza and S. pneumonia to reduce cases of otitis media, and avoiding or protecting against excessive noise exposure. Education on the perils of hazardous of noise exposure increases the use of hearing protectors.
There are a number of devices that can improve hearing in those who are hearing impaired or deaf or allow people with these conditions to better manage in society. Hearing aids, which amplify the incoming sound, will improve hearing ability, but nothing can restore normal hearing. Cochlear implants artificially stimulate the cochlear nerve by providing an electric impulse substitution for the firing of hair cells. Cochlear implants are not only expensive, but require sophisticated programming in conjunction with training for effectiveness. Cochlear implant recipients may be at higher risk for meningitis.